This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-252781, filed Aug. 23, 2000; No. 2000-374621, filed Dec. 8, 2000; and No. 2001-209735, filed Jul. 10, 2001, the entire contents of all of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electron gun assembly and more specifically to the structure of cathodes built in the electron gun assembly.
2. Description of the Related Art
An electron gun assembly used in general color cathode ray tubes comprises an electron beam generating section for generating three electron beams from cathodes arranged in a horizontal direction and a main lens section for accelerating the three electron beams and focusing them onto the phosphor screen. The electron beam generating section is constructed from at least three cathodes, a first electrode, and a second electrode. The cathodes are supplied with drive voltages synchronized with a video signal. The intensity of the electron beams (currents) emitted from the cathodes is controlled by the drive voltages.
One of visual characteristics required of color cathode ray tubes is that the picture quality is little subject to variation regardless of the intensity of electron beams (currents).
In general, increasing the beam current, i.e., increasing the electron beam intensity, causes the size of beam spots on the phosphor screen to be increased. This increase in the spot size causes the picture quality to deteriorate. One way to improve the deterioration in picture quality resulting from the increased spot size is to reduce the apparent spot size through the use of a commonly used velocity modulation coil (hereinafter referred to as a VM coil).
The VM coil is mounted externally around the neck of the tube. The VM coil is supplied with currents in synchronization with the rise and fall of a brightness signal so as to produce a very small deflection of the beams fast at the rise of the brightness signal but slow at the fall. As a consequence, the picture contrast is increased at the rise and fall of the brightness signal and the apparent spot size is reduced.
The current flowing in the VM coil depends on the magnitude of the drive voltage. At low beam currents, i.e., when the electron beam intensity is low, the current in the VM coil is also low, in which case the spot size little varies in the horizontal direction. On the other hand, for high beam currents, i.e., when the electron beam intensity is high, a high current flows in the VM coil, which results in a significant reduction in the spot size in the horizontal direction. The spot size is reduced only in the direction of deflection of electron beams by the deflection yoke, i.e., in the horizontal direction. The spot size in the vertical direction is not reduced. That is, an increase in the spot size in the vertical direction resulting from an increase in cathode current cannot be controlled.
Here, a description is given of the reason why an increase in the cathode current results in an increase in the spot size.
To increase the beam current from the cathode, the drive voltage to the cathode is increased. By so doing, the potential penetration is increased, so that the electron loading area in the cathode surface expands. As a result, the number of electrons emitted from the cathode (current) increases. An increase in the beam current and an expansion in the electron loading area cause the size of a virtual object point relative to the main lens to increase, resulting in the increased spot size on the phosphor screen.
With increasing beam current, the angle of divergence of an electron beam will also increase, causing the position of the virtual object point (the position of the object point which is seen by the main lens) to shift toward the phosphor screen. The forward shifting of the virtual object point changes the focusing voltage which keeps the electron beam spot in focus on the phosphor screen.
In general, the focusing voltage for video signals is constant. With increasing beam current, the beam spot on the phosphor screen becomes defocused gradually and the spot size increases.
With increase in the beam current, the space charges repelling effect at the crossover point of electron beam is enhanced, causing the size of the virtual object point to be increased and the virtual object point to shift toward the screen. As a result, the spot is increased in size as described previously.
Thus, when the beam current changes from a low value to a high value, the spot size on the phosphor screen increases, causing a degradation in picture definition.
One way to reduce the spot size at high beam currents is to reduce the diameter of the first electrode to thereby reduce the size of the virtual object point. However, this approach, while allowing the spot size at high beam currents to be reduced, cannot control variations in the spot size due to beam current variations. That is, this approach not only reduces the spot size at high beam currents but also reduces the spot size at low beam currents excessively. This may produce a degradation in picture quality, such as moire.
That is, with the way to reduce the diameter of the first electrode, it is impossible to control variations in the spot size due to beam current variations.
In Japanese Patent Application KOKAI Publications Nos. 11-120931 and 11-283487, there are disclosed techniques by which the electron loading area is restricted according to beam current variations to thereby control an increase in the spot size at high beam currents. According to these techniques, the cathode is formed with a core emitter in the center of its surface, a non-emission region around the core emitter, and a circumferential emitter around the non-emission region. The circumferential emitter is only left from a manufactural point of view and in practice it does not contribute to the emission of electrons.
Another cathode structure is such that there are provided a region suitable for emitting electrons in the center of the cathode surface (a region low in work function) and a region not suitable for emitting electrons around the center region (a region high in work function).
Those publications describe that good picture quality can be obtained by restricting the electron loading area to the center of the cathode surface, reducing the amount of circumferential beams containing many aberration components, and forming beam spots with little halo. With this method, however, the electron emission capability of the cathode is significantly degraded at high beam current time and, in producing a high beam current, the drive voltage has to be set considerably higher than usual. This increases the burden on drive circuitry, which leads to an increase in the cost of the drive circuitry and a reduction in the reliability of the drive circuitry.
As described above, in order to provide good picture quality, it is required to make the spot size on the phosphor screen little vary with varying beam current. Optimization of the sensitivity of the VM coil allows an increase in the spot size in the horizontal direction when the beam current changes from a low value to a high value to be compensated for. However, an increase in the spot size in the vertical direction cannot be compensated for. Such problems cannot also be solved by making the electron beam generating section smaller in size. That is, with the conventional methods, it is difficult to optimize the spot size in both the horizontal and vertical directions regardless of the beam current variations.
With the method in which the electron loading area is restricted to the center of the cathode, it is possible to suppress an increase in the spot size when the beam current changes from a low value to a high value, but the drive voltage has to be increased significantly, which increases the burden on drive circuitry, increases the cost thereof, and reduces the reliability thereof.
It is therefore an object of the present invention to provide an electron gun assembly and a cathode ray tube equipped with the gun assembly can be provided which allow an increase in the burden on drive circuitry to be controlled, an increase in the beam spot size in the horizontal and vertical directions on the phosphor screen with increasing beam current to be controlled, and high definition to be obtained.
According to an aspect of the present invention there is provided an electron gun assembly having an electron beam generating section which generates an electron beam and a main lens which accelerates the electron beam and focus it onto a target, wherein the electron beam generating section includes a cathode having an electron emitting surface and the surface of the cathode is divided into at least three regions of first, second and third regions which are different in electron emission capability, the first region being arranged in the center of the surface of the cathode, the second region being arranged on opposite sides of the first region in a first direction, and the third region being arranged on opposite sides of the first region in a second direction.
According to another aspect of the present invention there is provided a cathode ray tube apparatus including an electron gun assembly having an electron beam generating section which generates three electron beams in a horizontal direction and a main lens which accelerates the electron beams and focus them onto a phosphor screen, deflection yoke which deflects the three electron beams to scan across the phosphor screen in the horizontal and vertical directions, and velocity modulation coil which modulates the velocity of the electron beams, wherein the electron beam generating section includes three horizontally aligned cathodes and each having an electron emitting surface, a first electrode, and a second electrode which are arranged in this order in the direction in which the electron beams travel, and the surface of each of the cathodes is divided into at least three regions of first, second and third regions which have different electron emission capabilities, the first region being arranged in the center of the surface of the cathode, the second region being arranged on opposite sides of the first region in the horizontal direction, and the third region being arranged on opposite sides of the first region in the vertical direction.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.